" he was keeping track of time. It was nearly two hours since he had last looked at his watch, but he knew what time it was to within about twenty seconds. It was an old skill, born of many long wakeful nights on active service. When you're waiting for something to happen, you close your body down like a beach house in winter and you let your mind lock onto the steady pace of the passing seconds. It's like suspended animation. It saves energy and it lifts the responsibility for your heartbeat away from your unconscious brain and passes it on to some kind of a hidden clock. Makes a huge black space for thinking in. But it keeps you just awake enough to be reach for whatever you need to be ready for. And it means you always know what time it is. "

― Lee Child , Die Trying (Jack Reacher, #2)